This invention relates to several methods of ensuring oil return from the various system components to the compressor under various operational conditions, and preventing oil pump out from the compressor causing subsequent compressor damage.
Refrigerant cycles are utilized to provide cooling or heating. A refrigerant is compressed by a compressor, and then moved through a series of heat exchangers, connection lines and expansion devices.
There are many distinct configurations and arrangements of refrigerant cycles. One of the options is the use of multi-circuit refrigerant systems. A multi-circuit system has at least two circuits, each including a compressor and the associated heat exchangers, connection lines and expansion devices for conditioning a common area. The circuits, each including a compressor, condenser, expansion device, and evaporator are controlled to maintain a desired temperature in an environment to be cooled or heated.
Multi-circuit systems are prone to oil pump out under conditions where the amount of cooling required is just above of what one circuit can deliver. In this case, the system must be shut off frequently to compensate for the excessive supply of cold air generated by two circuits operating at the same time. Frequent start-stops can cause oil to be pumped out from the compressor reducing system efficiency by logging excessive amounts of oil in heat exchangers and potentially leading to compressor failure.
Another condition that can exist in both multi-circuit and single circuit systems that can lead to oil pump out is a low mass flow through the evaporator. If the mass flow is reduced below a certain level, the vapor can no longer carry the oil back to the compressor, again leading to oil pump out. The problem can be further aggravated by excessive vapor superheat entering the compressor, as high superheat leads to boiling off refrigerant from oil, increasing oil viscosity and causing the oil to “stick” to heat exchanger inner tube surfaces. Thus, the need exists to improve oil return under the above-mentioned conditions.
A control unit for the multi-circuit system separately controls all circuits, or some of the circuits to provide cooling. In the prior art, the control unit intermittently will shut down all circuits once sufficient cooling had been achieved. Alternatively, in the prior art, the control unit will sometimes shut down just some circuits while keeping the other ones operating when less cooling demand is placed on the overall system.
The present invention is intended to address the above-referenced problems that were present in the prior art control schemes.